The oil, gas and petrochemical industry desires to efficiently obtain hydrocarbons and process the hydrocarbons to produce desired products. Refining processes involve upgrading, converting or separating hydrocarbons (e.g., crude oil) into different streams, such as gases, light naphtha, heavy naphtha, kerosene, diesel, atmospheric gas oil, asphalt, petroleum coke and heavy hydrocarbons or fuel oil. Similarly, natural gas may be converted into industrial fuel gas, liquefied natural gas (LNG), ethane, propane, liquefied petroleum gas (LPG), and natural gas liquids (NGLs). The oil and gas processes are also often integrated with petrochemical systems to convert refinery streams into chemical products such as ethylene, propylene or polyolefins.
To convert hydrocarbon feeds into petrochemical or basic chemicals, chemical conversion processes may be utilized. These processes typically involve using thermal or catalytic reactors or furnaces to produce reactive hydrocarbon products, such as acetylene, ethylene or propylene in different proportions. As an example, steam cracking reactors are commonly utilized to convert the hydrocarbon feed into ethylene and acetylene, which may be further processed into various chemical products. The steam cracking reactors are utilized because they provide feed flexibility by being able to utilize gas (e.g., ethane) and liquid (e.g., naphtha) feeds. However, these feeds due to their high hydrogen content do not typically require hydrogen to manage reactor products from the process.
Historically, the oil and gas refineries utilize the higher value distillates from the hydrocarbon feed, which are typically fungible fuels, such as mogas, natural gas and diesel. As a result, the petrochemical refineries utilize the remaining fractions, such as ethane, propane, naphtha and virgin gas oil, in their processes. However, few chemical conversion processes are able to directly employ natural gas or the lower value refinery feeds, such as aromatic gas oils or fuel oils. As such, there is a need for a process that can produce ethylene and acetylene from different feeds, such as advantaged feeds (e.g., natural gas and/or aromatic gas oils, for example).
To process these feeds, high-severity conditions (e.g., more severe operating conditions, such as higher temperatures) are generally involved to produce products having a higher value than the feed. High-severity conditions enable methane cracking and aromatic ring cracking, which do not occur at appreciable rates at typical low-severity conditions (e.g., conventional steam cracking conditions). At high-severity conditions, the primary products of thermal chemical conversion processes are acetylene and ethylene along with hydrogen (H2) and coke, which may vary in proportion depending on the temperatures, pressures, residence times and feed type utilized in the conversion process. Low-severity conditions may be still be used to convert higher hydrogen content refinery byproduct streams. At lower severity conditions, saturates may be converted to ethylene, propylene and butenes and alkyl aromatics may be converted to benzene, toluene and gasoline blend stock. Although high-severity operating conditions typically yield predominately acetylenes and hydrogen, acetylene may be further hydrogenated to ethylene and ultimately converted polyethylene or other derivatives using conventional technology.
High-severity and low-severity conversion processes are typically based on different pyrolysis reactors, which may include pyrolysis alone or integrated with combustion chemistry. That is, the reactors may include pyrolysis chemistry (e.g., thermochemical decomposition of feed at elevated temperatures in the absence of oxygen) alone or in combination with combustion chemistry (i.e., exothermic chemical reactions between a fuel and an oxidant). These pyrolysis reactors can be divided into different types: partial combustion that burns part of the pyrolysis feed, indirect combustion that involves contacting the pyrolysis feed with combustion products, arc process that generate the electric arc or plasma to crack the pyrolysis feed (e.g., U.S. Pat. No. 1,860,624), and thermal pyrolysis. Each of these pyrolysis types differs in the means of generating and transferring the heat for the pyrolysis, but can be broadly characterized as low-severity or high-severity.
Thermal pyrolysis reactors involve heating a solid material (e.g., by combustion) and using the heated solid material to crack the pyrolysis feed. In the thermal pyrolysis processes, the combustion products are typically maintained separate from the pyrolysis products. This pyrolysis technique involves various different types of reactors, such as a furnace (e.g., as used in steam cracking), a regenerative reactor (e.g., as used in the Wulff process) and others. For instance, thermal pyrolysis reactors are described in various references, such as U.S. Pat. Nos. 7,138,047 and 7,119,240. U.S. Pat. No. 7,119,240 describes an exemplary process for the conversion of natural gas into ethylene. In this process, natural gas is cracked in a furnace, actively quenched, and processed in a hydrogenation reactor to produce ethylene. As another example, U.S. Pat. No. 7,138,047 describes another steam cracking process that mixes a hydrocarbon feed with a dilution steam, flashing the mixture, and vaporizing a portion of the mixture in a pyrolysis reactor. In the process, the pyrolysis feed is passed through tubes in the radiant section of a pyrolysis reactor to crack the pyrolysis feed without contaminating it with combustion products. However, due to the nature of a tubular (metal) furnace, steam cracking is limited to effective cracking temperatures of below 1000° C. and residence times of greater than or equal to (≧) 100 milliseconds (ms), which does not allow conversion of either methane or aromatics, thereby limiting the feedstock selection. In addition, energy or furnace heat not used in cracking is partially lost in the furnace flue gas or in the quench, as products are quickly cooled to stop undesired reactions.
The “Wulff” reactor, as described in the IHS, SRI Consulting's Process Economics Program “Acetylene” Report Number 16 (1966) and 16A (1982) along with U.S. Pat. Nos. 2,319,679; 2,678,339; 2,692,819; 3,024,094, and 3,093,697, uses a reverse-flow pyrolysis reactor, which is operated at temperatures of less than (<) 1400° C., to produce olefins and alkynes, such as acetylene. The pyrolysis feed is heated by refractories which have previously been heated by combustion reactions. The pyrolysis feed is cracked, and then cooled outside of the reactor. The relatively slow quenching is a characteristic of the Wulff process that leads to coke and soot formation from using inefficient indirect heat transfer (e.g., from checker brick). Coke formation in the reactor provides fuel during the combustion cycle and excess coke or soot may be alleviated by using a light feed, i.e., a hydrocarbon containing a high proportion of hydrogen. However, because the indirect heat transfer limits the rate of heat input in the Wulff process, certain pyrolysis feeds, such as methane, may not be economically processed, which limits the feed flexibility for this process.
Further, these reactors have been dismissed as not being useful for the conversion of natural gas (or fuel oils) into acetylene or ethylene. That is, the inefficient refractories limit heat transfer (both for adding heat necessary for pyrolysis and for removing heat necessary for quenching). As a result, the Wulff reactors typically involve cracking temperatures below 1400° C. and involve the use of more expensive feeds, such as ethane, propane and naphtha. In addition, the poor heat transfer limits lead to greater soot generation resulting in poorer selectivity to desired products.
Moreover, low-severity thermal processes, such as steam cracking or the Wulff process, are unable to significantly convert methane, which is a hydrogen rich hydrocarbon that yields excess hydrogen when cracked. Likewise, low-severity processes are unable to react severely hydrogen deficient feeds, such as aromatic rings, that require excess hydrogen or hydrogen rich feeds as a co-reactant. High-severity conditions are required to make hydrogen (H) either as molecular gas or to make the hydrogen content in a feed an active participant in the reaction process. High-severity operating conditions yield higher concentrations of a hydrogen radical intermediate. Thermal and arc processes may utilize the high-severity hydrogen reactivity because the hydrogen is utilized with the feed, while partial combustion and indirect combustion processes are not suited to utilize this type of reaction because the reactor products are contaminated with high levels of combustion products (CO, CO2 and H2O), which make it more difficult to take advantage of high-severity hydrogen reactivity.
Although pyrolysis reactors, such as thermal and arc pyrolysis reactors, may be used to convert hydrocarbons into useful products, such as acetylene and ethylene, improved reactions are desired which can make use of a broader range of feeds. Accordingly, it is desirable to provide a process that manages hydrogen in the conversion of hydrocarbon feeds into olefins in an enhanced manner.